Certain materials exhibit a property known as the photoelectric effect that causes them to absorb photons of light and release electrons. When these free electrons are captured, electric current results. Photovoltaic (PV) power generation employs solar panels composed of a number of solar cells containing a PV material. Materials presently used for PVs comprise of mono crystalline silicon, polycrystalline silicon, amorphous silicon, cadmium telluride, and copper indium gallium selenide/sulfide. Due to the growing demand for renewable energy sources, the manufacturing of solar cells and PV arrays has advanced considerably in recent years. Further, driven by advances in technology and increases in manufacturing scale and sophistication, the cost of PV has declined steadily since the first solar cells were manufactured.
When more power is required than a single cell can deliver, cells are electrically connected together to form PV modules or solar panels. A single module is sufficient to power an emergency telephone, but for a house or a power plant, the modules must be arranged in multiple arrays.
PV cells require protection from the environment and are usually packaged tightly behind a glass sheet. The possibility of partial shading of PV arrays in terrestrial applications due to shadows of buildings, trees and clouds is a distinct reality. The un-shaded parts of a solar array will be illuminated more than the shaded regions of the array. The lowering of the intensity of solar illumination on the shaded solar cells causes a reduction in the overall power generation from a PV source. The short circuit current, Sic reduces significantly during shading whereas the open circuit voltage Voc does not change much. The other factors leading to the mismatch in characteristics of PV modules are temperature differences between the PV modules and manufacturing tolerances.
Interconnecting individual PV modules in series causes their voltages to add up while the current remains the same in all the modules. During partial shading conditions, this forces the PV modules generating lower current to operate in the reverse biased region of the Current-Voltage (I-V) characteristics which leads to large thermal dissipation and eventual module/cell damage. To prevent this, module bypass diodes are connected in parallel with each module. They conduct before the PV cells of a particular module get reverse biased and thereby prevent damage due to thermal runaway. When the bypass diodes conduct, power from the entire module is lost since the PV module voltage is clamped to the drop across the forward biased bypass diode. The PV array's Power-Voltage (P-V) curve shows multiple peaks and the overall power output of the PV array (source) decreases drastically. Under these circumstances, some of the popular Maximum Power Point (MPP) tracking algorithms require appropriate modifications to bypass the unwanted local maxima and bring the operating point close to the global MPP thereby increasing the complexity of the MPP tracking scheme.
Maximum power point trackers may implement different algorithms and switch between them based on the operating conditions of the array. MPP algorithms are necessary because PV arrays have a non-linear voltage-current characteristic with a unique point where the power produced is maximum. This point depends on the temperature of the panels and on the irradiance conditions. Both conditions change continuously during the day and are also different depending upon the season of the year. Furthermore, irradiation can change rapidly due to changing atmospheric conditions such as clouds. It is very important to track the MPP accurately under all possible conditions so that the maximum available power is always obtained.
One of the prominent MPP tracking scheme is the Distributed Maximum Power Point (DMPP) tracking scheme. DMPP is an MPP scheme that has a dedicated MPP tracker for each PV module or for each PV cell as opposed to a centralized MPP tracker for the entire PV array. A DMPP tracking scheme does away with the need for module bypass diodes. They are effective during situations of uniform illumination of all PV modules as well as during partial shading of some modules. In some schemes of DMPP tracking, the PV modules are isolated from each other. A dedicated MPP tracker realized with a non-isolated conventional DC-DC converter of either buck, boost or buck-boost topology is associated with each PV module. The outputs of these MPP trackers are usually connected in series to form a DC bus. Since an MPP tracker is dedicated to each PV module, each tracker can independently optimize the power flow from its source. A single partially shaded PV module can deliver a reduced power rather than being bypassed by a diode. The dedicated MPP tracking DC-DC converters may even be integrated into the PV modules to form a compact unit.
However, there are limitations to this scheme. The output current of the series connected MPP trackers must be equal. During partial shading of some PV modules, the current from these modules are lower than that of the well illuminated modules. Depending on the difference in the illumination of the series connected modules, the voltage requirements of the subsequent stages and the topology of the MPP tracker, there are situations where some partially illuminated PV modules may not deliver the full power that they are capable of generating. In another scheme for DMPP tracking, a DC-DC converter that is dedicated to each series connected PV module injects an equalization current across the PV module across which it is connected while maintaining the module voltage at the maximum power point. Here the partially shaded modules can deliver the full power that they are capable of generating irrespective of the illumination it receives.
In a currently existing system, the PV module current equalization scheme is described as “generation control circuit”. In one method, based on an isolated multi output DC-DC converter, precise module voltage and equalization current control is not possible.
In another existing method, a multi stage chopper is used with accurate control of the module voltage and equalization current. The disadvantage with the multi stage chopper is the lack of modularity since modules cannot be added or removed without adjustments in the design. Previous approaches make use of an isolated fly back converter for current equalization. A method of current equalization in series connected PV modules with a non-isolated bidirectional DC-DC converter of buck-boost or Cuk topology for each adjacent PV panel pair can shuffle power up or down the PV array.
Conventional DC-DC converters that make use of magnetics have been used for DMPP tracking schemes earlier. These DC-DC converters comprise of power handling inductors and transformers which result in low power densities and high realization cost.
In an existing method of maximum power point tracking, a DC-DC converter is connected across a solar array and is used as a maximum power point tracker to supply power to a DC-AC inverter or a DC load. The limitation of this method is that it makes use of conventional DC-DC converters which result in low power densities and high realization cost. In another existing method of maximum power point tracking, a pulse powered super capacitor is electrically connected between a photo voltaic panel and a DC-DC converter. The super capacitor with low internal resistance and fast response, serves as a steady state input/output energy storing device. The disadvantage of this method is that it is not ideal for integration with each PV module.
SC DC-DC converters accomplish power conversion with the help of capacitors that are electronically switched between the input power source and the output load. The most distinguishing feature of SC converters is the complete absence of power handling inductors and transformers leading to high power densities of up to 23 W/in3 and low realization cost. SC converters with efficiencies in excess of 95% have been realized under certain operating conditions. Hence, they are ideal for integration with the PV module and PV cell. Since, they do not require any form of tuning, they are ideal for large-scale production.